The design of fluid power systems often reflects a compromise between efficiency and the combination of complexity and response time. For example, increased efficiency is often gained at a cost of both increased complexity and decreased response time.
Some of the least complex and fastest responding fluid power systems generate fluid power with fixed displacement pumps that are sized to satisfy a demand for a given flow rate at a system pressure. Any excess flow that is not required to sustain the system pressure is exhausted by a relief valve but is otherwise available at the system pressure to satisfy a demand for additional flow. Energy losses are calculated as a product of the volume of exhausted fluid and its drop in pressure. Additional losses are associated with throttling that may be required to satisfy a demand for reduced pressure by a load.
Efficiency can be improved by adding a so-called "priority unloading" valve that maintains a predetermined compensating differential pressure across a restrictor between the fixed displacement pump and a load. The compensating differential pressure across the restrictor is maintained by exhausting any excess flow before it reaches the restrictor. Output pressure of the pump is limited to the sum of the compensating differential pressure across the restrictor and an instant load pressure demand.
Any excess flow that is not required to sustain the compensating differential pressure across the restrictor is exhausted at the output pressure of the pump, which may be significantly less than the system pressure limit maintained by the relief valve. Throttling losses are also limited by the compensating differential pressure. Although the excess flow is available to satisfy a demand for additional flow, response time may be decreased by the reduced pressure at which the excess flow is available.
A further improvement in efficiency can be made by replacing the fixed displacement pump and priority unloading valve with a pressure compensated variable displacement pump. The output capacity of the variable displacement pump is varied to sustain the system pressure. Although no excess flow is produced, considerable throttling may be necessary to satisfy a demand for a reduced pressure by a load. Instant demands for increased pressure are satisfied by reduced throttling, but the output capacity of the pump must be varied to sustain the pressure increase.
Generally, the highest efficiencies are exhibited by so-called "load sensing" systems in which the output capacity of a variable displacement pump is controlled to maintain a predetermined operating differential pressure across a flow control valve. No excess flow is produced and throttling losses across the flow control valve are limited by the operating differential pressure. However, neither additional flow nor additional pressure is available to significantly reduce response time over the time required to vary the output capacity of the variable displacement pump.
Nonetheless, even the most efficient load sensing systems waste considerable energy in branch lines that convey fluid to separate loads having unequal pressure demands. Although load sensing systems can limit throttling losses in the highest pressure branch to the operating differential pressure, all of the other branches are supplied at the same output pressure of the variable displacement pump and require additional throttling. Such energy losses are increased by either reduced pressure or increased flow demands in the lower pressure branches.
Similar energy losses also occur in branches supplied by fixed displacement pumps. Typically, all of the branches are supplied at the predetermined output pressure of the fixed displacement pump, and throttling losses in each branch depend upon the amount of flow in each branch and upon the difference between the predetermined output pressure of the pump and the individual load pressure demands of each branch. Significant disparities in flow rate and pressure between branches produce considerable energy losses.
Flow dividers have been used in branch circuits to apportion different amounts of flow between the branches. Some flow dividers are made up of pressure compensated restrictors that divide the flow into fixed proportions independently of inlet pressure. Although throttling losses at flow control valves in the branch circuits are reduced, the total losses in each branch, combining the throttling losses at the flow control valves with the throttling losses at the pressure compensated restrictors, remain the same.
However, gangs of fixed size displacers (e.g., gear pump/motor devices) have also been used to divide flow between branches in proportion to the respective capacities of the displacers. Although the displacers maintain flows in fixed proportions, a reduction in outlet pressure of one displacer, so operating as a motor, is converted into an increase in outlet pressure of another displacer, so operating as a pump. The variation in outlet pressures distributes fluid power in different proportions to the branches, but the total power delivered to all of the branches remains constant. Any reduction in the total power demand of the branches with respect to a constant power delivered to the displacers is lost as wasted energy.